In the field of speech synthesis, in particular based on the concatenation of sound segments for making up the desired phrase, the demand arises to represent the sound material used in the synthesis process in a compact manner. Code Excited Linear Prediction (CELP) is a well-known technique for representing a speech signal in a compact manner, and is characterized by the adoption of a method, known as Analysis by Synthesis (A-b-S), that consists in separating the speech signal into excitation and vocal tract components, coding the excitation and linear prediction coefficients (LPCs) for the vocal tract component using an index that points to a series of representations stored in a codebook. The selection of the best index for the excitation and for the vocal tract is chosen by comparing the original signal with the reconstructed signal. For a complete description of the CELP technique reference may be made to Wai C. Chu, Speech Coding Algorithms, ISBN 0-471-37312-5, p. 299-324. Modified versions of the CELP are instead disclosed in US 2005/197833, US 2005/096901, and US2006/206317. FIG. 1 shows a block diagram of the CELP technique for speech signal coding, where the vocal tract and the glottal source are modeled by an impulse source (excitation), referenced by F1-1, and by a variant-time digital filter (synthesis filter), referenced by F1-2.